The present invention relates to a magnetron for use in a microwave application apparatus such as an electronic oven.
In general, as shown in FIG. 5, a magnetron built into an electronic oven as a microwave oscillation device comprises a vacuum tube unit 1 arranged at a center, a plurality of radiating fins 2 arranged at a circumference of the vacuum tub unit 1, a pair of annular magnets 3 arranged concentrically to the vacuum tube unit 1, frame shaped yokes 4 and 5 for magnetically connecting the annular magnets 3, and a filter circuit unit 7. In addition, the vacuum tube unit 1 comprises an anode assembly 11, and a cathode assembly 21 built on the central axis of the anode assembly 11.
The anode assembly 11 comprises a substantially cylindrical anode tube body 12, an even number (N) of plate shaped vanes fixedly mounted on the anode tube body and radially from an inner circumference of the anode tube body 12 to a central axis to be spaced apart from an cathode assembly 21, two large and small strap rings 15 and 16 arranged at an end of a tube axis direction of the pate shaped vanes 13 for alternatively connecting for the respective plate shape vanes 13 for electrical short, and an antenna 17 connected to the plate vanes for outputting microwave, as shown in FIGS. 6 and 7.
In addition, the cathode assembly 21 a coil shaped filament 22 arranged at the center thereof, and end parts 23 and 24 connected to both ends of the filament 22, and a cathode supporting lid 25 connected to the filament 22 through these end parts 23 and 24, as shown in FIG. 5 (for example, see Patent Document 1).
The magnetron as mentioned above applies heat on the filament 22, and applies a high DC voltage between the filament 22 and the plate shaped vanes 13. Therefore, electrons radiated from the filament 22 to the plate shaped vanes 13 receives the effect of electromagnetic field that perpendiculars to a operational space 31 between the plate shapes vanes 123 and the filament 22, rotates around the filament 22, faces the plate shaped vanes 13 of the anode assembly 11, and produces an interaction with a minute microwave generated in a cavity resonator 33 divided by the even number of plate shaped vanes 13. Thus, a large microwave is generated in the cavity resonator 33 to output the generated microwave from the antenna 17.
A frequency of the microwave generated in the cavity resonator 33 is determined by an inductance L consisting of an inner circumferential wall of the anode tube body that forms the cavity resonator 33 and facing plate shaped vanes 13, and a capacitance C in combination with a capacitance Cr of the cavity resonator 33 consisting of the interrelated plate shaped vanes 13 and the anode assembly 12, and a capacitance Cs consisting of facing portions of the plate shaped vanes 13 and the strap rings 15 and 16. In general, the resonating frequency fr is represented as the following equation.fr=1/{2π(LC)1/2}  (1)
The frequency is oscillated most strong and stably among the magnetron oscillation types and becomes a so-called π mode oscillation frequency of an inverse phase between the adjacent cavity resonators, and a main function of two large and small strap rings 15 and 16 that alternatively connect the plate shaped vanes 13 to make an electrical short-circuit is to maintain the stability of the π mode oscillation.
However, in the magnetron, N cavity resonators divided by N plate shaped vanes 13 are electrically coupled between each other, so that when the plate shaped vanes 13 are electrically short-circuited by the two large and small strap rings 15 and 16 alternatively, the oscillation with N/2 of different frequencies is performed.
For example, when the number N of plate vanes 13 is 10 so that the number of the cavity resonators 33 divided by the plate shaped vanes 13 is 10, a fundamental mode has 5 oscillation modes from N/2, which represent N/2 mode, N/2-1 mode, N/2-2 mode, N/2-3 mode and N/2-4 mode, referred to as “the π mode”.
Therefore, in the π mode, oscillation can be made most strongly and stably under the operation conditions such as the frequency and the anode voltage. However, oscillation frequency in the N/2-1 mode adjacent to the π mode is close to the π mode oscillation frequency, so that even when the operation condition is changed very little, the oscillation is made from the π mode to N/2-1 mode, leading to an unstable phenomenon such as a mode jump.
Therefore, in order to set N/2-1 mode oscillation frequency apart from the π mode oscillation frequency, a ratio of capacitance Cr of the cavity resonator 33 formed by the respective plane shaped vanes 13 and the anode tube body 12 to capacitance Cs of the strap rings made of facing portions of the respective strap rings 15 and 16 and of the plate shaped panels is set to be large. But, a method in which the strap rings 15 and 16 are not all arranged in symmetry and a portion thereof is disconnected is proposed (for example, see pp. 163 to 166 of non-Patent Document 1).
In addition, to respond to the recently worldwide request for energy saving, there is a strong need of a highly efficient magnetron.
To achieve the highly efficient magnetron, high magnetic field, the number of split anodes and the small diameter of the anode and cathode are required, the distance between any two of the plate shaped vanes 13 becomes short (see pp. 172 to 177 of the afore-mentioned non-Patent Document 1).
Therefore, even when the distances of arrangement between the plate shaped vanes 13 with each other become short, a method of forming tapered surfaces 13a at both sides of the end portions of the respective plate shaped vanes 13 was proposed, as shown in FIG. 8, in order to secure a predetermined separation distance between the adjacent plate shaped vanes 13 (for example, see Patent Document 2).
[Patent Document 1] Japanese Patent Laid-Open No. 11-233036
[Patent Document 1] Japanese Patent Laid-Open No. 60-127638
[Non-Patent Document 1] ‘Microwave Vacuum Tube’ published by wireless technology industry Employee Training Association on December 1956.
Therefore, the capacitance Cr of the cavity resonator 33 consisting of the adjacent plate shaped vanes 13 and the anode tube body 12 is approximately determined by the capacitance Cg of end portions of the respective plate shaped vanes 13 which is closest to each other. Thus, as shown in FIG. 8(a), when the facing area of the end portions of the respective plate shaped vane 13 which are closest to each other is S, and the distance between the facing surface is d, Cr can be represented as the following equation 2.Cr≈Cg=ε×S/d   (2)
Thus, according to the construction where the taper surface 13a is arranged at both sides of the end portions of the respective plate shaped vanes 13 as described above, in fact, such a large separation distance cannot be secured. As a result, the capacitance Cr of the cavity resonator 3 becomes large.
Further, FIG. 8(b) shows an equivalent circuit diagram of FIG. 8(a).
To secure a predetermined value of a composition capacitance C based on the above equation 1, the capacitance Cr of the resonant cavity 33 should be large and a ratio of the capacitance Cs of the strap rings should be small.
As a result, a ratio of the capacitance Cs to the capacitance Cr is determined to be large so that the N/2-1 mode oscillation frequency is set apart from the π mode oscillation frequency, leading to a problem on instability for the operation condition due to any of the mode jumps. Furthermore, it is difficult to achieve both the high efficiency and the stable operation.
In addition, to guarantee a large separation distance between the plate shaped vanes 13, the respective vanes may be formed to have small thickness. However, as the thickness is small, it will not have a heat capacity as a magnetron.